<!DOCTYPE HTML>
<html lang="zh-CN">


<head>
    <meta charset="utf-8">
    <meta name="keywords" content="Java中的泛型使用, 大虫のBlog">
    <meta name="description" content="电子科技大学 | 信软学院 | 软件工程">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=no">
    <meta name="renderer" content="webkit|ie-stand|ie-comp">
    <meta name="mobile-web-app-capable" content="yes">
    <meta name="format-detection" content="telephone=no">
    <meta name="apple-mobile-web-app-capable" content="yes">
    <meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
    <!-- Global site tag (gtag.js) - Google Analytics -->


    <title>Java中的泛型使用 | 大虫のBlog</title>
    <link rel="icon" type="image/png" href="/favicon.png">

    <link rel="stylesheet" type="text/css" href="/libs/awesome/css/all.css">
    <link rel="stylesheet" type="text/css" href="/libs/materialize/materialize.min.css">
    <link rel="stylesheet" type="text/css" href="/libs/aos/aos.css">
    <link rel="stylesheet" type="text/css" href="/libs/animate/animate.min.css">
    <link rel="stylesheet" type="text/css" href="/libs/lightGallery/css/lightgallery.min.css">
    <link rel="stylesheet" type="text/css" href="/css/matery.css">
    <link rel="stylesheet" type="text/css" href="/css/my.css">

    <script src="/libs/jquery/jquery.min.js"></script>

<meta name="generator" content="Hexo 6.2.0"></head>




<body>
    <header class="navbar-fixed">
    <nav id="headNav" class="bg-color nav-transparent">
        <div id="navContainer" class="nav-wrapper container">
            <div class="brand-logo">
                <a href="/" class="waves-effect waves-light">
                    
                    <img src="/medias/logo.png" class="logo-img" alt="LOGO">
                    
                    <span class="logo-span">大虫のBlog</span>
                </a>
            </div>
            

<a href="#" data-target="mobile-nav" class="sidenav-trigger button-collapse"><i class="fas fa-bars"></i></a>
<ul class="right nav-menu">
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="/" class="waves-effect waves-light">
      
      <i class="fas fa-home" style="zoom: 0.6;"></i>
      
      <span>首页</span>
    </a>
    
  </li>
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="/tags" class="waves-effect waves-light">
      
      <i class="fas fa-tags" style="zoom: 0.6;"></i>
      
      <span>标签</span>
    </a>
    
  </li>
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="/categories" class="waves-effect waves-light">
      
      <i class="fas fa-bookmark" style="zoom: 0.6;"></i>
      
      <span>分类</span>
    </a>
    
  </li>
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="/archives" class="waves-effect waves-light">
      
      <i class="fas fa-archive" style="zoom: 0.6;"></i>
      
      <span>归档</span>
    </a>
    
  </li>
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="/about" class="waves-effect waves-light">
      
      <i class="fas fa-user-circle" style="zoom: 0.6;"></i>
      
      <span>关于</span>
    </a>
    
  </li>
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="/contact" class="waves-effect waves-light">
      
      <i class="fas fa-comments" style="zoom: 0.6;"></i>
      
      <span>留言板</span>
    </a>
    
  </li>
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="/friends" class="waves-effect waves-light">
      
      <i class="fas fa-address-book" style="zoom: 0.6;"></i>
      
      <span>友情链接</span>
    </a>
    
  </li>
  
  <li class="hide-on-med-and-down nav-item">
    
    <a href="" class="waves-effect waves-light">

      
      <i class="fa fa-list" style="zoom: 0.6;"></i>
      
      <span>其他</span>
      <i class="fas fa-chevron-down" aria-hidden="true" style="zoom: 0.6;"></i>
    </a>
    <ul class="sub-nav menus_item_child ">
      
      <li>
        <a target="_blank" rel="noopener" href="https://console.leancloud.cn">
          
          <i class="fa-solid fa-link" style="margin-top: -20px; zoom: 0.6;"></i>
          
          <span>LeanCloud 应用</span>
        </a>
      </li>
      
      <li>
        <a target="_blank" rel="noopener" href="https://www.tidio.com/panel">
          
          <i class="fa-solid fa-link" style="margin-top: -20px; zoom: 0.6;"></i>
          
          <span>TIDIO 聊天系统</span>
        </a>
      </li>
      
      <li>
        <a target="_blank" rel="noopener" href="https://www.coderutil.com">
          
          <i class="fa-solid fa-link" style="margin-top: -20px; zoom: 0.6;"></i>
          
          <span>CoderUtil 工具</span>
        </a>
      </li>
      
    </ul>
    
  </li>
  
  <li>
    <a href="#searchModal" class="modal-trigger waves-effect waves-light">
      <i id="searchIcon" class="fas fa-search" title="搜索" style="zoom: 0.85;"></i>
    </a>
  </li>
</ul>


<div id="mobile-nav" class="side-nav sidenav">

    <div class="mobile-head bg-color">
        
        <img src="/medias/logo.png" class="logo-img circle responsive-img">
        
        <div class="logo-name">大虫のBlog</div>
        <div class="logo-desc">
            
            电子科技大学 | 信软学院 | 软件工程
            
        </div>
    </div>

    

    <ul class="menu-list mobile-menu-list">
        
        <li class="m-nav-item">
	  
		<a href="/" class="waves-effect waves-light">
			
			    <i class="fa-fw fas fa-home"></i>
			
			首页
		</a>
          
        </li>
        
        <li class="m-nav-item">
	  
		<a href="/tags" class="waves-effect waves-light">
			
			    <i class="fa-fw fas fa-tags"></i>
			
			标签
		</a>
          
        </li>
        
        <li class="m-nav-item">
	  
		<a href="/categories" class="waves-effect waves-light">
			
			    <i class="fa-fw fas fa-bookmark"></i>
			
			分类
		</a>
          
        </li>
        
        <li class="m-nav-item">
	  
		<a href="/archives" class="waves-effect waves-light">
			
			    <i class="fa-fw fas fa-archive"></i>
			
			归档
		</a>
          
        </li>
        
        <li class="m-nav-item">
	  
		<a href="/about" class="waves-effect waves-light">
			
			    <i class="fa-fw fas fa-user-circle"></i>
			
			关于
		</a>
          
        </li>
        
        <li class="m-nav-item">
	  
		<a href="/contact" class="waves-effect waves-light">
			
			    <i class="fa-fw fas fa-comments"></i>
			
			留言板
		</a>
          
        </li>
        
        <li class="m-nav-item">
	  
		<a href="/friends" class="waves-effect waves-light">
			
			    <i class="fa-fw fas fa-address-book"></i>
			
			友情链接
		</a>
          
        </li>
        
        <li class="m-nav-item">
	  
		<a href="javascript:;">
			
				<i class="fa-fw fa fa-list"></i>
			
			其他
			<span class="m-icon"><i class="fas fa-chevron-right"></i></span>
		</a>
            <ul  style="background:  ;" >
              
                <li>

                  <a target="_blank" rel="noopener" href="https://console.leancloud.cn " style="margin-left:75px">
				  
				   <i class="fa fa-solid fa-link" style="position: absolute;left:50px" ></i>
			      
		          <span>LeanCloud 应用</span>
                  </a>
                </li>
              
                <li>

                  <a target="_blank" rel="noopener" href="https://www.tidio.com/panel " style="margin-left:75px">
				  
				   <i class="fa fa-solid fa-link" style="position: absolute;left:50px" ></i>
			      
		          <span>TIDIO 聊天系统</span>
                  </a>
                </li>
              
                <li>

                  <a target="_blank" rel="noopener" href="https://www.coderutil.com " style="margin-left:75px">
				  
				   <i class="fa fa-solid fa-link" style="position: absolute;left:50px" ></i>
			      
		          <span>CoderUtil 工具</span>
                  </a>
                </li>
              
            </ul>
          
        </li>
        
        
        <li><div class="divider"></div></li>
        <li>
            <a href="https://gitee.com/yczlab" class="waves-effect waves-light" target="_blank">
                <i class="fab fa-github-square fa-fw"></i>Fork Me
            </a>
        </li>
        
    </ul>
</div>


        </div>

        
            <style>
    .nav-transparent .github-corner {
        display: none !important;
    }

    .github-corner {
        position: absolute;
        z-index: 10;
        top: 0;
        right: 0;
        border: 0;
        transform: scale(1.1);
    }

    .github-corner svg {
        color: #0f9d58;
        fill: #fff;
        height: 64px;
        width: 64px;
    }

    .github-corner:hover .octo-arm {
        animation: a 0.56s ease-in-out;
    }

    .github-corner .octo-arm {
        animation: none;
    }

    @keyframes a {
        0%,
        to {
            transform: rotate(0);
        }
        20%,
        60% {
            transform: rotate(-25deg);
        }
        40%,
        80% {
            transform: rotate(10deg);
        }
    }
</style>

<a href="https://gitee.com/yczlab" class="github-corner tooltipped hide-on-med-and-down" target="_blank"
   data-tooltip="Fork Me" data-position="left" data-delay="50">
    <svg viewBox="0 0 250 250" aria-hidden="true">
        <path d="M0,0 L115,115 L130,115 L142,142 L250,250 L250,0 Z"></path>
        <path d="M128.3,109.0 C113.8,99.7 119.0,89.6 119.0,89.6 C122.0,82.7 120.5,78.6 120.5,78.6 C119.2,72.0 123.4,76.3 123.4,76.3 C127.3,80.9 125.5,87.3 125.5,87.3 C122.9,97.6 130.6,101.9 134.4,103.2"
              fill="currentColor" style="transform-origin: 130px 106px;" class="octo-arm"></path>
        <path d="M115.0,115.0 C114.9,115.1 118.7,116.5 119.8,115.4 L133.7,101.6 C136.9,99.2 139.9,98.4 142.2,98.6 C133.8,88.0 127.5,74.4 143.8,58.0 C148.5,53.4 154.0,51.2 159.7,51.0 C160.3,49.4 163.2,43.6 171.4,40.1 C171.4,40.1 176.1,42.5 178.8,56.2 C183.1,58.6 187.2,61.8 190.9,65.4 C194.5,69.0 197.7,73.2 200.1,77.6 C213.8,80.2 216.3,84.9 216.3,84.9 C212.7,93.1 206.9,96.0 205.4,96.6 C205.1,102.4 203.0,107.8 198.3,112.5 C181.9,128.9 168.3,122.5 157.7,114.1 C157.9,116.9 156.7,120.9 152.7,124.9 L141.0,136.5 C139.8,137.7 141.6,141.9 141.8,141.8 Z"
              fill="currentColor" class="octo-body"></path>
    </svg>
</a>
        
    </nav>

</header>

    



<div class="bg-cover pd-header post-cover" style="background-image: url('/medias/featureimages/14.jpg')">
    <div class="container" style="right: 0px;left: 0px;">
        <div class="row">
            <div class="col s12 m12 l12">
                <div class="brand">
                    <h1 class="description center-align post-title">Java中的泛型使用</h1>
                </div>
            </div>
        </div>
    </div>
</div>




<main class="post-container content">

    
    <link rel="stylesheet" href="/libs/tocbot/tocbot.css">
<style>
    #articleContent h1::before,
    #articleContent h2::before,
    #articleContent h3::before,
    #articleContent h4::before,
    #articleContent h5::before,
    #articleContent h6::before {
        display: block;
        content: " ";
        height: 100px;
        margin-top: -100px;
        visibility: hidden;
    }

    #articleContent :focus {
        outline: none;
    }

    .toc-fixed {
        position: fixed;
        top: 64px;
    }

    .toc-widget {
        width: 345px;
        padding-left: 20px;
    }

    .toc-widget .toc-title {
        padding: 35px 0 15px 17px;
        font-size: 1.5rem;
        font-weight: bold;
        line-height: 1.5rem;
    }

    .toc-widget ol {
        padding: 0;
        list-style: none;
    }

    #toc-content {
        padding-bottom: 30px;
        overflow: auto;
    }

    #toc-content ol {
        padding-left: 10px;
    }

    #toc-content ol li {
        padding-left: 10px;
    }

    #toc-content .toc-link:hover {
        color: #42b983;
        font-weight: 700;
        text-decoration: underline;
    }

    #toc-content .toc-link::before {
        background-color: transparent;
        max-height: 25px;

        position: absolute;
        right: 23.5vw;
        display: block;
    }

    #toc-content .is-active-link {
        color: #42b983;
    }

    #floating-toc-btn {
        position: fixed;
        right: 15px;
        bottom: 76px;
        padding-top: 15px;
        margin-bottom: 0;
        z-index: 998;
    }

    #floating-toc-btn .btn-floating {
        width: 48px;
        height: 48px;
    }

    #floating-toc-btn .btn-floating i {
        line-height: 48px;
        font-size: 1.4rem;
    }
</style>
<div class="row">
    <div id="main-content" class="col s12 m12 l9">
        <!-- 文章内容详情 -->
<div id="artDetail">
    <div class="card">
        <div class="card-content article-info">
            <div class="row tag-cate">
                <div class="col s7">
                    
                    <div class="article-tag">
                        
                            <a href="/tags/Java/">
                                <span class="chip bg-color">Java</span>
                            </a>
                        
                    </div>
                    
                </div>
                <div class="col s5 right-align">
                    
                    <div class="post-cate">
                        <i class="fas fa-bookmark fa-fw icon-category"></i>
                        
                            <a href="/categories/Java/" class="post-category">
                                Java
                            </a>
                        
                    </div>
                    
                </div>
            </div>

            <div class="post-info">
                
                <div class="post-date info-break-policy">
                    <i class="far fa-calendar-minus fa-fw"></i>发布日期:&nbsp;&nbsp;
                    2021-01-13
                </div>
                

                
                <div class="post-date info-break-policy">
                    <i class="far fa-calendar-check fa-fw"></i>更新日期:&nbsp;&nbsp;
                    2022-07-31
                </div>
                

                
                <div class="info-break-policy">
                    <i class="far fa-file-word fa-fw"></i>文章字数:&nbsp;&nbsp;
                    3k
                </div>
                

                
                <div class="info-break-policy">
                    <i class="far fa-clock fa-fw"></i>阅读时长:&nbsp;&nbsp;
                    13 分
                </div>
                

                
                    <div id="busuanzi_container_page_pv" class="info-break-policy">
                        <i class="far fa-eye fa-fw"></i>阅读次数:&nbsp;&nbsp;
                        <span id="busuanzi_value_page_pv"></span>
                    </div>
				
            </div>
        </div>
        <hr class="clearfix">

        
        <!-- 是否加载使用自带的 prismjs. -->
        <link rel="stylesheet" href="/libs/prism/prism.css">
        

        

        <div class="card-content article-card-content">
            <div id="articleContent">
                <p>  </p>
<h2 id="引言："><a href="#引言：" class="headerlink" title="引言："></a>引言：</h2><blockquote>
<p>从JDK1.5以后，Java引入了“参数化类型（Parameterized type）”的概念，允许我们在创建集合时再指定集合元素的类型，正如：List&lt;String&gt;，这表明该List只能保存字符串类型的对象。</p>
</blockquote>
<h3 id="一、泛型的设计背景"><a href="#一、泛型的设计背景" class="headerlink" title="一、泛型的设计背景"></a>一、泛型的设计背景</h3><p>  集合容器类在设计阶段&#x2F;声明阶段不能确定这个容器到底实际存的是什么类型的对象，所以在JDK1.5之前只能把元素类型设计为Object，JDK1.5之后使用泛型来解决。因为这个时候除了元素的类型不确定，其他的部分是确定的，例如关于这个元素如何保存，如何管理等是确定的，因此此时把元素的类型设计成一个参数，这个类型参数叫做泛型。Collection&lt;E&gt;，List&lt;E&gt;，ArrayList&lt;E&gt; 这个&lt;E&gt;就是类型参数，即泛型。</p>
<h3 id="二、泛型的概念"><a href="#二、泛型的概念" class="headerlink" title="二、泛型的概念"></a>二、泛型的概念</h3><p>  所谓泛型，就是允许在定义类、接口时通过一个标识表示类中某个属性的类型或者是某个方法的返回值及参数类型。这个类型参数将在使用时（例如，继承或实现这个接口，用这个类型声明变量、创建对象时）确定（即传入实际的类型参数，也称为类型实参）。JDK1.5改写了集合框架中的全部接口和类，为这些接口、类增加了泛型支持，从而可以在声明集合变量、创建集合对象时传入类型实参。</p>
<table>
<thead>
<tr>
<th>有泛型时</th>
<th>无泛型时</th>
</tr>
</thead>
<tbody><tr>
<td><img src="/posts/8255cc5e/1.png"></td>
<td><img src="/posts/8255cc5e/2.png"></td>
</tr>
</tbody></table>
<h3 id="三、在集合中使用泛型"><a href="#三、在集合中使用泛型" class="headerlink" title="三、在集合中使用泛型"></a>三、在集合中使用泛型</h3><ol>
<li>集合接口或集合类在jdk5.0时都修改为带泛型的结构。</li>
<li>在实例化集合类时，可以指明具体的泛型类型</li>
<li>指明完以后，在集合类或接口中凡是定义类或接口时，内部结构（比如：方法、构造器、属性等）使用到类的泛型的位置，都指定为实例化的泛型类型。比如：add(E e)  —&gt;实例化以后：add(Integer e)</li>
<li><strong>注意点：泛型的类型必须是类，不能是基本数据类型。需要用到基本数据类型的位置，拿包装类替换</strong></li>
<li>如果实例化时，没有指明泛型的类型。<strong>默认类型为java.lang.Object类型</strong>。<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">GenericTest</span> <span class="token punctuation">&#123;</span>
    <span class="token annotation punctuation">@Test</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">tset1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token comment">//Map&lt;String,Integer> map = new HashMap&lt;String,Integer>();</span>
        <span class="token comment">//jdk7新特性：类型推断</span>
        <span class="token class-name">Map</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">,</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"Tom"</span><span class="token punctuation">,</span><span class="token number">87</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"Jerry"</span><span class="token punctuation">,</span><span class="token number">87</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        map<span class="token punctuation">.</span><span class="token function">put</span><span class="token punctuation">(</span><span class="token string">"Jack"</span><span class="token punctuation">,</span><span class="token number">67</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">//map.put(123,"ABC");//会报错</span>

        <span class="token comment">//泛型的嵌套</span>
        <span class="token class-name">Set</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Map<span class="token punctuation">.</span>Entry</span><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">,</span><span class="token class-name">Integer</span><span class="token punctuation">></span><span class="token punctuation">></span></span> entry <span class="token operator">=</span> map<span class="token punctuation">.</span><span class="token function">entrySet</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Map<span class="token punctuation">.</span>Entry</span><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token punctuation">></span><span class="token punctuation">></span></span> iterator <span class="token operator">=</span> entry<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token keyword">while</span><span class="token punctuation">(</span>iterator<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
            <span class="token class-name">Map<span class="token punctuation">.</span>Entry</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">,</span> <span class="token class-name">Integer</span><span class="token punctuation">></span></span> e <span class="token operator">=</span> iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token class-name">String</span> key <span class="token operator">=</span> e<span class="token punctuation">.</span><span class="token function">getKey</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token class-name">Integer</span> value <span class="token operator">=</span> e<span class="token punctuation">.</span><span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>key <span class="token operator">+</span> <span class="token string">"----"</span> <span class="token operator">+</span> value<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre></li>
</ol>
<h3 id="四、自定义泛型结构：泛型类、泛型接口；泛型方法"><a href="#四、自定义泛型结构：泛型类、泛型接口；泛型方法" class="headerlink" title="四、自定义泛型结构：泛型类、泛型接口；泛型方法"></a>四、自定义泛型结构：泛型类、泛型接口；泛型方法</h3><h4 id="泛型类、泛型接口："><a href="#泛型类、泛型接口：" class="headerlink" title="泛型类、泛型接口："></a>泛型类、泛型接口：</h4><pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Order</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span>
    <span class="token class-name">String</span> orderName<span class="token punctuation">;</span>
    <span class="token keyword">int</span> orderId<span class="token punctuation">;</span>

    <span class="token class-name">T</span> orderT<span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token class-name">Order</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>

    <span class="token punctuation">&#125;</span>

    <span class="token keyword">public</span> <span class="token class-name">Order</span><span class="token punctuation">(</span><span class="token class-name">String</span> orderName<span class="token punctuation">,</span> <span class="token keyword">int</span> <span class="token class-name">OrderId</span><span class="token punctuation">,</span> <span class="token class-name">T</span> orderT<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>orderName <span class="token operator">=</span> orderName<span class="token punctuation">;</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>orderId <span class="token operator">=</span> <span class="token class-name">OrderId</span><span class="token punctuation">;</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>orderT <span class="token operator">=</span> orderT<span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">getOrderT</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> orderT<span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setOrderT</span><span class="token punctuation">(</span><span class="token class-name">T</span> orderT<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">this</span><span class="token punctuation">.</span>orderT <span class="token operator">=</span> orderT<span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> <span class="token string">"Order&#123;"</span> <span class="token operator">+</span>
                <span class="token string">"orderName='"</span> <span class="token operator">+</span> orderName <span class="token operator">+</span> <span class="token char">'\''</span> <span class="token operator">+</span>
                <span class="token string">", orderId="</span> <span class="token operator">+</span> orderId <span class="token operator">+</span>
                <span class="token string">", orderT="</span> <span class="token operator">+</span> orderT <span class="token operator">+</span>
                <span class="token char">'&#125;'</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>

<pre class="language-java" data-language="java"><code class="language-java"><span class="token annotation punctuation">@Test</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    <span class="token comment">//如果定义了泛型类，实例化没有指明类的泛型，则认为此泛型类型为Object类型</span>
    <span class="token comment">//要求：如果大家定义了类是带泛型的，建议在实例化时要指明类的泛型</span>
    <span class="token class-name">Order</span> order <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Order</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    order<span class="token punctuation">.</span><span class="token function">setOrderT</span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    order<span class="token punctuation">.</span><span class="token function">setOrderT</span><span class="token punctuation">(</span><span class="token string">"ABC"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token comment">//建议：实例化时指明类的泛型</span>
    <span class="token class-name">Order</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> order1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Order</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token string">"orderAA"</span><span class="token punctuation">,</span><span class="token number">1001</span><span class="token punctuation">,</span><span class="token string">"order:AA"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    order1<span class="token punctuation">.</span><span class="token function">setOrderT</span><span class="token punctuation">(</span><span class="token string">"AA:hello"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">&#125;</span></code></pre>

<ol>
<li>泛型类可能有多个参数，此时应将多个参数一起放在尖括号内。比如：&lt;E1,E2,E3&gt;</li>
<li>泛型类的构造器如下：public GenericClass(){}。而下面是错误的：public GenericClass&lt;E&gt;(){}</li>
<li>实例化后，操作原来泛型位置的结构必须与指定的泛型类型一致。</li>
<li>泛型不同的引用不能相互赋值。尽管在编译时ArrayList&lt;String&gt;和ArrayList&lt;Integer&gt;是两种类型，但是，在运行时只有一个ArrayList被加载到JVM中。</li>
<li>泛型如果不指定，将被擦除，泛型对应的类型均按照Object处理，但不等价于Object。<strong>经验：泛型要使用一路都用。要不用，一路都不要用。</strong></li>
<li>如果泛型结构是一个接口或抽象类，则不可创建泛型类的对象。</li>
<li>jdk1.7，泛型的简化操作：ArrayList&lt;Fruit&gt; flist &#x3D; new ArrayList&lt;&gt;();自动类型推断。</li>
<li>泛型的指定中不能使用基本数据类型，可以使用包装类替换。</li>
<li>在类&#x2F;接口上声明的泛型，在本类或本接口中即代表某种类型，可以作为非静态属性的类型、非静态方法的参数类型、非静态方法的返回值类型。<strong>但在静态方法中不能使用类的泛型。</strong></li>
<li><strong>异常类不能是泛型的</strong></li>
<li>不能使用new E[]来实例化一个泛型对象。但是可以：E[] elements &#x3D; (E[])new Object[capacity]; 参考ArrayList源码中的声明：Object[] elementData，而非泛型参数类型数组。</li>
<li><strong>父类中有泛型，子类可以选择保留泛型，也可以选择指定泛型类型：</strong><blockquote>
<p>子类不保留父类的泛型：按需实现</p>
<blockquote>
<ul>
<li>没有类型，擦除</li>
<li>具体类型（子类在继承带泛型的父类时，如果指明了泛型类型。则实例化子类对象时，不再需要指明泛型，前提是子类没有自定义其他类型的泛型）</li>
</ul>
</blockquote>
<p>子类保留父类的泛型：泛型子类</p>
<blockquote>
<ul>
<li>全部保留 (public class ArrayList&lt;E&gt; extends AbstractList&lt;E&gt;)</li>
<li>部分保留</li>
</ul>
</blockquote>
</blockquote>
</li>
</ol>
<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">class</span> <span class="token class-name">Father</span><span class="token generics"><span class="token punctuation">&lt;</span>T1<span class="token punctuation">,</span> T2<span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span>
<span class="token punctuation">&#125;</span>

<span class="token comment">// 子类不保留父类的泛型</span>
<span class="token comment">// 1）没有类型 擦除</span>
<span class="token keyword">class</span> <span class="token class-name">Son1</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">A</span><span class="token punctuation">,</span> <span class="token class-name">B</span><span class="token punctuation">></span></span> <span class="token keyword">extends</span> <span class="token class-name">Father</span> <span class="token punctuation">&#123;</span><span class="token comment">//等价于class Son1&lt;A, B> extends Father&lt;Object, Object>&#123;</span>

<span class="token punctuation">&#125;</span>

<span class="token comment">// 2）具体类型</span>
<span class="token keyword">class</span> <span class="token class-name">Son2</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">A</span><span class="token punctuation">,</span> <span class="token class-name">B</span><span class="token punctuation">></span></span> <span class="token keyword">extends</span> <span class="token class-name">Father</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span>
<span class="token punctuation">&#125;</span>

<span class="token comment">//子类保留父类的泛型</span>
<span class="token comment">// 1）全部保留</span>
<span class="token keyword">class</span> <span class="token class-name">Son3</span><span class="token generics"><span class="token punctuation">&lt;</span>T1<span class="token punctuation">,</span> T2<span class="token punctuation">,</span> <span class="token class-name">A</span><span class="token punctuation">,</span> <span class="token class-name">B</span><span class="token punctuation">></span></span> <span class="token keyword">extends</span> <span class="token class-name">Father</span><span class="token generics"><span class="token punctuation">&lt;</span>T1<span class="token punctuation">,</span> T2<span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span>

<span class="token comment">// 2）部分保留</span>
<span class="token keyword">class</span> <span class="token class-name">Son4</span><span class="token generics"><span class="token punctuation">&lt;</span>T2<span class="token punctuation">,</span> <span class="token class-name">A</span><span class="token punctuation">,</span> <span class="token class-name">B</span><span class="token punctuation">></span></span> <span class="token keyword">extends</span> <span class="token class-name">Father</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> T2<span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span>

<span class="token punctuation">&#125;</span></code></pre>
<p><strong>注意：</strong><br><img src="/posts/8255cc5e/3.png"></p>
<h4 id="泛型方法"><a href="#泛型方法" class="headerlink" title="泛型方法"></a>泛型方法</h4><ol>
<li>方法也可以被泛型化，不管此时所在类是不是泛型类。在泛型方法中可以定义泛型参数，此时，参数的类型就是传入数据的类型。<strong>泛型方法可以是静态方法。</strong></li>
<li>泛型方法的格式：<blockquote>
<p>[访问权限] &lt;泛型&gt; 返回类型 方法名([泛型标识 参数名称]) 抛出的异常</p>
</blockquote>
</li>
</ol>
<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">class</span> <span class="token class-name">Creature</span><span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span>
<span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token keyword">extends</span> <span class="token class-name">Creature</span><span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span>
<span class="token keyword">class</span> <span class="token class-name">Man</span> <span class="token keyword">extends</span> <span class="token class-name">Person</span><span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span>
<span class="token keyword">class</span> <span class="token class-name">PersonTest</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span> <span class="token keyword">extends</span> <span class="token class-name">Person</span><span class="token punctuation">></span></span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token class-name">T</span> t<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>t<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token function">test</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">test</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Man</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">test</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Creature</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//报错</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>

<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">GenericTest</span> <span class="token punctuation">&#123;</span>
    <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token keyword">void</span> <span class="token function">fromArrayToCollection</span><span class="token punctuation">(</span><span class="token class-name">T</span><span class="token punctuation">[</span><span class="token punctuation">]</span> a<span class="token punctuation">,</span> <span class="token class-name">Collection</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> c<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">T</span> o <span class="token operator">:</span> a<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            c<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span>o<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token annotation punctuation">@Test</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> ao <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token number">100</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token class-name">Collection</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">></span></span> co <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">fromArrayToCollection</span><span class="token punctuation">(</span>ao<span class="token punctuation">,</span> co<span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> as <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">[</span><span class="token number">20</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
        <span class="token class-name">Collection</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> cs <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">fromArrayToCollection</span><span class="token punctuation">(</span>as<span class="token punctuation">,</span> cs<span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">Collection</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Double</span><span class="token punctuation">></span></span> cd <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">//fromArrayToCollection(as, cd);//中T是Double类，但sa是String类型，编译错误。</span>
        <span class="token function">fromArrayToCollection</span><span class="token punctuation">(</span>as<span class="token punctuation">,</span> co<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//T是Object类型，sa是String类型，可以赋值成功。</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>

<h3 id="五、泛型类和泛型方法的使用情景"><a href="#五、泛型类和泛型方法的使用情景" class="headerlink" title="五、泛型类和泛型方法的使用情景"></a>五、泛型类和泛型方法的使用情景</h3><pre class="language-java" data-language="java"><code class="language-java"><span class="token comment">/**
 * DAO：database access object
 */</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">DAO</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span><span class="token comment">//表的共性操作的DAO</span>
    <span class="token comment">/* 添加一条记录 */</span>
    <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">add</span><span class="token punctuation">(</span><span class="token class-name">T</span> t<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token comment">/* 删除一条记录 */</span>
    <span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">remove</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token comment">/* 修改一条记录 */</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">update</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">,</span> <span class="token class-name">T</span> t<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>

    <span class="token punctuation">&#125;</span>

    <span class="token comment">/* 查询一条记录 */</span>
    <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">getIndex</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token comment">/* 查询多条记录 */</span>
    <span class="token keyword">public</span> <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">></span></span> <span class="token function">getForList</span><span class="token punctuation">(</span><span class="token keyword">int</span> index<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

	<span class="token comment">//泛型方法</span>
    <span class="token comment">/* 获取表中一共有多少条记录(int、long)? 获取最大的员工入职时间*/</span>
    <span class="token keyword">public</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">></span></span> <span class="token class-name">E</span> <span class="token function">getValue</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    	<span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>

<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">class</span> <span class="token class-name">Customer</span> <span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span>
<span class="token keyword">class</span> <span class="token class-name">Student</span> <span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span>

<span class="token keyword">class</span> <span class="token class-name">CustomerDao</span> <span class="token keyword">extends</span> <span class="token class-name">DAO</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Customer</span><span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span><span class="token comment">//只能操作一个表的DAO</span>
<span class="token keyword">class</span> <span class="token class-name">StudentDao</span> <span class="token keyword">extends</span> <span class="token class-name">DAO</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Student</span><span class="token punctuation">></span></span> <span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span><span class="token comment">//只能操作一个表的DAO</span></code></pre>

<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token class-name">DAOTest</span> <span class="token punctuation">&#123;</span>
	<span class="token annotation punctuation">@Test</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
    	<span class="token class-name">CustomerDao</span> dao1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">CustomerDao</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        dao1<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Customer</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Customer</span><span class="token punctuation">></span></span> list1 <span class="token operator">=</span> dao1<span class="token punctuation">.</span><span class="token function">getForList</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">StudentDao</span> dao2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">StudentDao</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">Student</span> student <span class="token operator">=</span> dao2<span class="token punctuation">.</span><span class="token function">getIndex</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>

<h3 id="五、泛型在继承上的体现"><a href="#五、泛型在继承上的体现" class="headerlink" title="五、泛型在继承上的体现"></a>五、泛型在继承上的体现</h3><pre class="language-java" data-language="java"><code class="language-java"><span class="token comment">/**
 * 1. 虽然类A是类B的父类，但是 G&lt;A> 和 G&lt;B> 二者不具备子父类关系，二者是并列关系。但是二者共同的父类是 G&lt;?>
 * 2. 类A是类B的父类，A&lt;G> 是 B&lt;G> 的父类
 *
 */</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">GenericTest</span> <span class="token punctuation">&#123;</span>
    <span class="token annotation punctuation">@Test</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test1</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">Object</span> obj <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token class-name">String</span> str <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        obj <span class="token operator">=</span> str<span class="token punctuation">;</span><span class="token comment">//不会报错（多态的体现）</span>

        <span class="token class-name">Object</span><span class="token punctuation">[</span><span class="token punctuation">]</span> arr1 <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> arr2 <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        arr1 <span class="token operator">=</span> arr2<span class="token punctuation">;</span><span class="token comment">//不会报错（也是多态得体现）</span>

        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">></span></span> list1 <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> list2 <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        list1 <span class="token operator">=</span> list2<span class="token punctuation">;</span><span class="token comment">//编译不过，会报错。此时的 list1 与 list2 不具有子父类关系</span>

        <span class="token function">show</span><span class="token punctuation">(</span>list1<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//不报错</span>
        <span class="token function">show</span><span class="token punctuation">(</span>list2<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//报错</span>

        <span class="token function">show1</span><span class="token punctuation">(</span>list1<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//不报错</span>
        <span class="token function">show1</span><span class="token punctuation">(</span>list2<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//不报错</span>
    <span class="token punctuation">&#125;</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">show</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">></span></span> list<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>

    <span class="token punctuation">&#125;</span>

    <span class="token keyword">public</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span> <span class="token keyword">extends</span> <span class="token class-name">Object</span><span class="token punctuation">></span></span> <span class="token keyword">void</span> <span class="token function">show1</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">></span></span> list<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>

    <span class="token punctuation">&#125;</span>

    <span class="token annotation punctuation">@Test</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test2</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> list1 <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token class-name">AbstractList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> list2 <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> list3 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        list1 <span class="token operator">=</span> list2<span class="token punctuation">;</span><span class="token comment">//不报错</span>
        list2 <span class="token operator">=</span> list3<span class="token punctuation">;</span><span class="token comment">//不报错</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>

<h3 id="六、通配符"><a href="#六、通配符" class="headerlink" title="六、通配符"></a>六、通配符</h3><ol>
<li><p>使用类型<strong>通配符：”？”</strong><br>比如：List&lt;?&gt; ，Map&lt;?,?&gt;<br>List&lt;?&gt; 是 List&lt;String&gt;、List&lt;Object&gt;等各种泛型List的父类。</p>
</li>
<li><p>读取List&lt;?&gt;的对象list中的元素时，永远是安全的，因为不管list的真实类型是什么，它包含的都是Object。</p>
</li>
<li><p>写入list&lt;?&gt;中的元素时，不行。因为我们不知道c的元素类型，我们不能向其中添加对象。但是，唯一的例外是null，它是所有类型的成员。</p>
<blockquote>
<p>Collection&lt;?&gt; c &#x3D; new ArrayList&lt;String&gt;();<br>c.add(new Object());&#x2F;&#x2F;编译时错误<br>  因为我们并不知道c的元素类型，我们不能向其中添加对象。add方法有类型参数E作为集合的元素类型。我们传给add的任何参数都必须是一个未知类型的子类。因为我们不知道那是什么类型，所以我们无法传任何东西进去。<br>  另一方面，我们可以调用get()方法并使用其返回值。返回值是一个未知的类型，但是我们知道，他总是一个Object</p>
</blockquote>
</li>
<li><p><strong>类A是类B的父类，G&lt;A&gt; 和 G&lt;B&gt;是没有子父类关系的，但二者共同父类是 G&lt;?&gt;</strong></p>
</li>
</ol>
<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">GenericTest</span> <span class="token punctuation">&#123;</span>
    <span class="token annotation punctuation">@Test</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> list <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span><span class="token comment">//通配符的使用</span>
        list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">//list.add(3);//编译不通过</span>
        list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">null</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">read</span><span class="token punctuation">(</span>list<span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span> l1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span> l2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        l1<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"哈哈哈"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        l2<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">14</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token function">read</span><span class="token punctuation">(</span>l1<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">read</span><span class="token punctuation">(</span>l2<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>

    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">read</span><span class="token punctuation">(</span><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> list<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span><span class="token comment">//通配符的使用</span>
        <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Object</span> obj <span class="token operator">:</span> list<span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>

        <span class="token class-name">Iterator</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">></span></span> iterator <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">iterator</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">while</span> <span class="token punctuation">(</span>iterator<span class="token punctuation">.</span><span class="token function">hasNext</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
            <span class="token class-name">Object</span> obj <span class="token operator">=</span> iterator<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">&#125;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>

<h4 id="有限制条件的通配符"><a href="#有限制条件的通配符" class="headerlink" title="有限制条件的通配符"></a>有限制条件的通配符</h4><ol>
<li><p>通配符指定上限</p>
<blockquote>
<p>上限extends：使用时指定的类型必须是继承某个类，或者实现某个接口，即 ≤</p>
</blockquote>
</li>
<li><p>通配符指定下限</p>
<blockquote>
<p>下限super：使用时指定的类型不能小于操作的类，即 ≥</p>
</blockquote>
</li>
<li><p>举例：</p>
<blockquote>
<ul>
<li>&lt;? extends Number&gt; 表示只允许泛型为Number及Number子类的引用调用</li>
<li>&lt;? super Number&gt; 表示只允许泛型为Number及Number父类的引用调用</li>
<li>&lt;? extends Comparable&gt; 表示只允许泛型为实现Comparable接口的实现类调用</li>
</ul>
</blockquote>
</li>
</ol>
<pre class="language-java" data-language="java"><code class="language-java"><span class="token keyword">class</span> <span class="token class-name">Person</span> <span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span>
<span class="token keyword">class</span> <span class="token class-name">Student</span> <span class="token keyword">extends</span> <span class="token class-name">Person</span> <span class="token punctuation">&#123;</span><span class="token punctuation">&#125;</span>

<span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">GenericTest</span> <span class="token punctuation">&#123;</span>
    <span class="token annotation punctuation">@Test</span>
    <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">&#123;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">Person</span><span class="token punctuation">></span></span> a <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">Person</span><span class="token punctuation">></span></span> b <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>

        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Student</span><span class="token punctuation">></span></span> list1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Student</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Person</span><span class="token punctuation">></span></span> list2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Person</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">></span></span> list3 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">></span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        a <span class="token operator">=</span> list1<span class="token punctuation">;</span>
        a <span class="token operator">=</span> list2<span class="token punctuation">;</span>
        <span class="token comment">//a = list3;//报错</span>

        <span class="token comment">//b = list1;//报错</span>
        b <span class="token operator">=</span> list2<span class="token punctuation">;</span>
        b <span class="token operator">=</span> list3<span class="token punctuation">;</span>

        <span class="token comment">//读取数据</span>
        a <span class="token operator">=</span> list1<span class="token punctuation">;</span>
        <span class="token class-name">Person</span> p <span class="token operator">=</span> a<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        b <span class="token operator">=</span> list2<span class="token punctuation">;</span>
        <span class="token class-name">Object</span> obj <span class="token operator">=</span> b<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token comment">//写入数据</span>
        <span class="token comment">//a.add(new Student());//报错。将?理解成未知类型，未知类型可能是Student的子类，所以会报错</span>
        <span class="token comment">//a.add(new Person());//报错。未知类型可能是Student，所以会报错</span>
        <span class="token comment">//a.add(new Object());//报错。未知类型可能的最大值为Person，所以会报错</span>

        <span class="token comment">//b.add(new Object());//报错。未知类型可能为Person，所以会报错</span>
        b<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Person</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        b<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">Student</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span></code></pre>


                
            </div>
            <hr/>

            

    <div class="reprint" id="reprint-statement">
        
            <div class="reprint__author">
                <span class="reprint-meta" style="font-weight: bold;">
                    <i class="fas fa-user">
                        文章作者:
                    </i>
                </span>
                <span class="reprint-info">
                    <a href="/about" rel="external nofollow noreferrer">YangChongZhi</a>
                </span>
            </div>
            <div class="reprint__type">
                <span class="reprint-meta" style="font-weight: bold;">
                    <i class="fas fa-link">
                        文章链接:
                    </i>
                </span>
                <span class="reprint-info">
                    <a href="http://yczlab.top/posts/8255cc5e.html">http://yczlab.top/posts/8255cc5e.html</a>
                </span>
            </div>
            <div class="reprint__notice">
                <span class="reprint-meta" style="font-weight: bold;">
                    <i class="fas fa-copyright">
                        版权声明:
                    </i>
                </span>
                <span class="reprint-info">
                    本博客所有文章除特別声明外，均采用
                    <a href="https://creativecommons.org/licenses/by/4.0/deed.zh" rel="external nofollow noreferrer" target="_blank">CC BY 4.0</a>
                    许可协议。转载请注明来源
                    <a href="/about" target="_blank">YangChongZhi</a>
                    !
                </span>
            </div>
        
    </div>

    <script async defer>
      document.addEventListener("copy", function (e) {
        let toastHTML = '<span>复制成功，请遵循本文的转载规则</span><button class="btn-flat toast-action" onclick="navToReprintStatement()" style="font-size: smaller">查看</a>';
        M.toast({html: toastHTML})
      });

      function navToReprintStatement() {
        $("html, body").animate({scrollTop: $("#reprint-statement").offset().top - 80}, 800);
      }
    </script>



            <div class="tag_share" style="display: block;">
                <div class="post-meta__tag-list" style="display: inline-block;">
                    
                        <div class="article-tag">
                            
                                <a href="/tags/Java/">
                                    <span class="chip bg-color">Java</span>
                                </a>
                            
                        </div>
                    
                </div>
                <div class="post_share" style="zoom: 80%; width: fit-content; display: inline-block; float: right; margin: -0.15rem 0;">
                    <link rel="stylesheet" type="text/css" href="/libs/share/css/share.min.css">
<div id="article-share">

    
    <div class="social-share" data-sites="twitter,facebook,google,qq,qzone,wechat,weibo,douban,linkedin" data-wechat-qrcode-helper="<p>微信扫一扫即可分享！</p>"></div>
    <script src="/libs/share/js/social-share.min.js"></script>
    

    

</div>

                </div>
            </div>
            
                <style>
    #reward {
        margin: 40px 0;
        text-align: center;
    }

    #reward .reward-link {
        font-size: 1.4rem;
        line-height: 38px;
    }

    #reward .btn-floating:hover {
        box-shadow: 0 6px 12px rgba(0, 0, 0, 0.2), 0 5px 15px rgba(0, 0, 0, 0.2);
    }

    #rewardModal {
        width: 320px;
        height: 350px;
    }

    #rewardModal .reward-title {
        margin: 15px auto;
        padding-bottom: 5px;
    }

    #rewardModal .modal-content {
        padding: 10px;
    }

    #rewardModal .close {
        position: absolute;
        right: 15px;
        top: 15px;
        color: rgba(0, 0, 0, 0.5);
        font-size: 1.3rem;
        line-height: 20px;
        cursor: pointer;
    }

    #rewardModal .close:hover {
        color: #ef5350;
        transform: scale(1.3);
        -moz-transform:scale(1.3);
        -webkit-transform:scale(1.3);
        -o-transform:scale(1.3);
    }

    #rewardModal .reward-tabs {
        margin: 0 auto;
        width: 210px;
    }

    .reward-tabs .tabs {
        height: 38px;
        margin: 10px auto;
        padding-left: 0;
    }

    .reward-content ul {
        padding-left: 0 !important;
    }

    .reward-tabs .tabs .tab {
        height: 38px;
        line-height: 38px;
    }

    .reward-tabs .tab a {
        color: #fff;
        background-color: #ccc;
    }

    .reward-tabs .tab a:hover {
        background-color: #ccc;
        color: #fff;
    }

    .reward-tabs .wechat-tab .active {
        color: #fff !important;
        background-color: #22AB38 !important;
    }

    .reward-tabs .alipay-tab .active {
        color: #fff !important;
        background-color: #019FE8 !important;
    }

    .reward-tabs .reward-img {
        width: 210px;
        height: 210px;
    }
</style>

<div id="reward">
    <a href="#rewardModal" class="reward-link modal-trigger btn-floating btn-medium waves-effect waves-light red">赏</a>

    <!-- Modal Structure -->
    <div id="rewardModal" class="modal">
        <div class="modal-content">
            <a class="close modal-close"><i class="fas fa-times"></i></a>
            <h4 class="reward-title">你的赏识是我前进的动力</h4>
            <div class="reward-content">
                <div class="reward-tabs">
                    <ul class="tabs row">
                        <li class="tab col s6 alipay-tab waves-effect waves-light"><a href="#alipay">支付宝</a></li>
                        <li class="tab col s6 wechat-tab waves-effect waves-light"><a href="#wechat">微 信</a></li>
                    </ul>
                    <div id="alipay">
                        <img src="/medias/reward/alipay.jpg" class="reward-img" alt="支付宝打赏二维码">
                    </div>
                    <div id="wechat">
                        <img src="/medias/reward/wechat.png" class="reward-img" alt="微信打赏二维码">
                    </div>
                </div>
            </div>
        </div>
    </div>
</div>

<script>
    $(function () {
        $('.tabs').tabs();
    });
</script>

            
        </div>
    </div>

    

    

    

    

    
        <style>
    .valine-card {
        margin: 1.5rem auto;
    }

    .valine-card .card-content {
        padding: 20px 20px 5px 20px;
    }

    #vcomments textarea {
        box-sizing: border-box;
        background: url("/medias/comment_bg.webp") 100% 100% no-repeat;
    }

    #vcomments p {
        margin: 2px 2px 10px;
        font-size: 1.05rem;
        line-height: 1.78rem;
    }

    #vcomments blockquote p {
        text-indent: 0.2rem;
    }

    #vcomments a {
        padding: 0 2px;
        color: #4cbf30;
        font-weight: 500;
        text-decoration: none;
    }

    #vcomments img {
        max-width: 100%;
        height: auto;
        cursor: pointer;
    }

    #vcomments ol li {
        list-style-type: decimal;
    }

    #vcomments ol,
    ul {
        display: block;
        padding-left: 2em;
        word-spacing: 0.05rem;
    }

    #vcomments ul li,
    ol li {
        display: list-item;
        line-height: 1.8rem;
        font-size: 1rem;
    }

    #vcomments ul li {
        list-style-type: disc;
    }

    #vcomments ul ul li {
        list-style-type: circle;
    }

    #vcomments table, th, td {
        padding: 12px 13px;
        border: 1px solid #dfe2e5;
    }

    #vcomments table, th, td {
        border: 0;
    }

    table tr:nth-child(2n), thead {
        background-color: #fafafa;
    }

    #vcomments table th {
        background-color: #f2f2f2;
        min-width: 80px;
    }

    #vcomments table td {
        min-width: 80px;
    }

    #vcomments h1 {
        font-size: 1.85rem;
        font-weight: bold;
        line-height: 2.2rem;
    }

    #vcomments h2 {
        font-size: 1.65rem;
        font-weight: bold;
        line-height: 1.9rem;
    }

    #vcomments h3 {
        font-size: 1.45rem;
        font-weight: bold;
        line-height: 1.7rem;
    }

    #vcomments h4 {
        font-size: 1.25rem;
        font-weight: bold;
        line-height: 1.5rem;
    }

    #vcomments h5 {
        font-size: 1.1rem;
        font-weight: bold;
        line-height: 1.4rem;
    }

    #vcomments h6 {
        font-size: 1rem;
        line-height: 1.3rem;
    }

    #vcomments p {
        font-size: 1rem;
        line-height: 1.5rem;
    }

    #vcomments hr {
        margin: 12px 0;
        border: 0;
        border-top: 1px solid #ccc;
    }

    #vcomments blockquote {
        margin: 15px 0;
        border-left: 5px solid #42b983;
        padding: 1rem 0.8rem 0.3rem 0.8rem;
        color: #666;
        background-color: rgba(66, 185, 131, .1);
    }

    #vcomments pre {
        font-family: monospace, monospace;
        padding: 1.2em;
        margin: .5em 0;
        background: #272822;
        overflow: auto;
        border-radius: 0.3em;
        tab-size: 4;
    }

    #vcomments code {
        font-family: monospace, monospace;
        padding: 1px 3px;
        font-size: 0.92rem;
        color: #e96900;
        background-color: #f8f8f8;
        border-radius: 2px;
    }

    #vcomments pre code {
        font-family: monospace, monospace;
        padding: 0;
        color: #e8eaf6;
        background-color: #272822;
    }

    #vcomments pre[class*="language-"] {
        padding: 1.2em;
        margin: .5em 0;
    }

    #vcomments code[class*="language-"],
    pre[class*="language-"] {
        color: #e8eaf6;
    }

    #vcomments [type="checkbox"]:not(:checked), [type="checkbox"]:checked {
        position: inherit;
        margin-left: -1.3rem;
        margin-right: 0.4rem;
        margin-top: -1px;
        vertical-align: middle;
        left: unset;
        visibility: visible;
    }

    #vcomments b,
    strong {
        font-weight: bold;
    }

    #vcomments dfn {
        font-style: italic;
    }

    #vcomments small {
        font-size: 85%;
    }

    #vcomments cite {
        font-style: normal;
    }

    #vcomments mark {
        background-color: #fcf8e3;
        padding: .2em;
    }

    #vcomments table, th, td {
        padding: 12px 13px;
        border: 1px solid #dfe2e5;
    }

    table tr:nth-child(2n), thead {
        background-color: #fafafa;
    }

    #vcomments table th {
        background-color: #f2f2f2;
        min-width: 80px;
    }

    #vcomments table td {
        min-width: 80px;
    }

    #vcomments [type="checkbox"]:not(:checked), [type="checkbox"]:checked {
        position: inherit;
        margin-left: -1.3rem;
        margin-right: 0.4rem;
        margin-top: -1px;
        vertical-align: middle;
        left: unset;
        visibility: visible;
    }
</style>

<div class="card valine-card" data-aos="fade-up">
    <div class="comment_headling" style="font-size: 20px; font-weight: 700; position: relative; padding-left: 20px; top: 15px; padding-bottom: 5px;">
        <i class="fas fa-comments fa-fw" aria-hidden="true"></i>
        <span>评论</span>
    </div>
    <div id="vcomments" class="card-content" style="display: grid">
    </div>
</div>

<script src="/libs/valine/av-min.js"></script>
<script src="/libs/valine/Valine.min.js"></script>
<script>
    new Valine({
        el: '#vcomments',
        appId: 'jJDtz1MXtl9cRxLcrXYvlvhd-9Nh9j0Va',
        appKey: 'UpF6BuVYYhInv6M8vXS0dWRt',
        pageSize: '10',
        notify: 'false' === 'true',
        verify: 'false' === 'true',
        visitor: 'false' === 'true',
        avatar: 'monsterid',
		recordIP: 'true',
		enableQQ: 'true',
		requiredFields: ["nick", "mail"],
        lang: 'zh-cn',
        placeholder: '请开始你的表演吧！\n注意博客留言要求填写昵称、邮箱'
    });
</script>

    

    

    

    

<article id="prenext-posts" class="prev-next articles">
    <div class="row article-row">
        
        <div class="article col s12 m6" data-aos="fade-up">
            <div class="article-badge left-badge text-color">
                <i class="fas fa-chevron-left"></i>&nbsp;上一篇</div>
            <div class="card">
                <a href="/posts/fa8d17db.html">
                    <div class="card-image">
                        
                        
                        <img src="/medias/featureimages/9.jpg" class="responsive-img" alt="Java中File类的使用">
                        
                        <span class="card-title">Java中File类的使用</span>
                    </div>
                </a>
                <div class="card-content article-content">
                    <div class="summary block-with-text">
                        
                              
引言：
java.io.File 是用来表示文件和文件目录路径的抽象形式，与平台无关。


File 能新建、删除、重命名文件和目录，但是File不能访问文件内容本身。如果需要访问文件内容本身，则需要使用输入&#x2F;输出流。
想要
                        
                    </div>
                    <div class="publish-info">
                        <span class="publish-date">
                            <i class="far fa-clock fa-fw icon-date"></i>2021-01-14
                        </span>
                        <span class="publish-author">
                            
                            <i class="fas fa-bookmark fa-fw icon-category"></i>
                            
                            <a href="/categories/Java/" class="post-category">
                                    Java
                                </a>
                            
                            
                        </span>
                    </div>
                </div>
                
                <div class="card-action article-tags">
                    
                    <a href="/tags/Java/">
                        <span class="chip bg-color">Java</span>
                    </a>
                    
                </div>
                
            </div>
        </div>
        
        
        <div class="article col s12 m6" data-aos="fade-up">
            <div class="article-badge right-badge text-color">
                下一篇&nbsp;<i class="fas fa-chevron-right"></i>
            </div>
            <div class="card">
                <a href="/posts/b78c789a.html">
                    <div class="card-image">
                        
                        
                        <img src="/medias/featureimages/6.jpg" class="responsive-img" alt="Java中的常用类常用方法">
                        
                        <span class="card-title">Java中的常用类常用方法</span>
                    </div>
                </a>
                <div class="card-content article-content">
                    <div class="summary block-with-text">
                        
                              
引言：
Java中常用类中的常用方法，如果能够记住些，对刷题还是有一定的帮助的。

一、Object类

public final native Class&lt;?&gt; getClass();
native方法，用于返回当前运行
                        
                    </div>
                    <div class="publish-info">
                            <span class="publish-date">
                                <i class="far fa-clock fa-fw icon-date"></i>2021-01-13
                            </span>
                        <span class="publish-author">
                            
                            <i class="fas fa-bookmark fa-fw icon-category"></i>
                            
                            <a href="/categories/Java/" class="post-category">
                                    Java
                                </a>
                            
                            
                        </span>
                    </div>
                </div>
                
                <div class="card-action article-tags">
                    
                    <a href="/tags/Java/">
                        <span class="chip bg-color">Java</span>
                    </a>
                    
                </div>
                
            </div>
        </div>
        
    </div>
</article>

</div>



<!-- 代码块功能依赖 -->
<script type="text/javascript" src="/libs/codeBlock/codeBlockFuction.js"></script>

<!-- 代码语言 -->

<script type="text/javascript" src="/libs/codeBlock/codeLang.js"></script>


<!-- 代码块复制 -->

<script type="text/javascript" src="/libs/codeBlock/codeCopy.js"></script>


<!-- 代码块收缩 -->

<script type="text/javascript" src="/libs/codeBlock/codeShrink.js"></script>


    </div>
    <div id="toc-aside" class="expanded col l3 hide-on-med-and-down">
        <div class="toc-widget card" style="background-color: white;">
            <div class="toc-title"><i class="far fa-list-alt"></i>&nbsp;&nbsp;目录</div>
            <div id="toc-content"></div>
        </div>
    </div>
</div>

<!-- TOC 悬浮按钮. -->

<div id="floating-toc-btn" class="hide-on-med-and-down">
    <a class="btn-floating btn-large bg-color">
        <i class="fas fa-list-ul"></i>
    </a>
</div>


<script src="/libs/tocbot/tocbot.min.js"></script>
<script>
    $(function () {
        tocbot.init({
            tocSelector: '#toc-content',
            contentSelector: '#articleContent',
            headingsOffset: -($(window).height() * 0.4 - 45),
            collapseDepth: Number('0'),
            headingSelector: 'h2, h3, h4'
        });

        // modify the toc link href to support Chinese.
        let i = 0;
        let tocHeading = 'toc-heading-';
        $('#toc-content a').each(function () {
            $(this).attr('href', '#' + tocHeading + (++i));
        });

        // modify the heading title id to support Chinese.
        i = 0;
        $('#articleContent').children('h2, h3, h4').each(function () {
            $(this).attr('id', tocHeading + (++i));
        });

        // Set scroll toc fixed.
        let tocHeight = parseInt($(window).height() * 0.4 - 64);
        let $tocWidget = $('.toc-widget');
        $(window).scroll(function () {
            let scroll = $(window).scrollTop();
            /* add post toc fixed. */
            if (scroll > tocHeight) {
                $tocWidget.addClass('toc-fixed');
            } else {
                $tocWidget.removeClass('toc-fixed');
            }
        });

        
        /* 修复文章卡片 div 的宽度. */
        let fixPostCardWidth = function (srcId, targetId) {
            let srcDiv = $('#' + srcId);
            if (srcDiv.length === 0) {
                return;
            }

            let w = srcDiv.width();
            if (w >= 450) {
                w = w + 21;
            } else if (w >= 350 && w < 450) {
                w = w + 18;
            } else if (w >= 300 && w < 350) {
                w = w + 16;
            } else {
                w = w + 14;
            }
            $('#' + targetId).width(w);
        };

        // 切换TOC目录展开收缩的相关操作.
        const expandedClass = 'expanded';
        let $tocAside = $('#toc-aside');
        let $mainContent = $('#main-content');
        $('#floating-toc-btn .btn-floating').click(function () {
            if ($tocAside.hasClass(expandedClass)) {
                $tocAside.removeClass(expandedClass).hide();
                $mainContent.removeClass('l9');
            } else {
                $tocAside.addClass(expandedClass).show();
                $mainContent.addClass('l9');
            }
            fixPostCardWidth('artDetail', 'prenext-posts');
        });
        
    });
</script>

    

</main>




    <footer class="page-footer bg-color">
    
        <link rel="stylesheet" href="/libs/aplayer/APlayer.min.css">
<style>
    .aplayer .aplayer-lrc p {
        
        display: none;
        
        font-size: 12px;
        font-weight: 700;
        line-height: 16px !important;
    }

    .aplayer .aplayer-lrc p.aplayer-lrc-current {
        
        display: none;
        
        font-size: 15px;
        color: #42b983;
    }

    
    .aplayer.aplayer-fixed.aplayer-narrow .aplayer-body {
        left: -66px !important;
    }

    .aplayer.aplayer-fixed.aplayer-narrow .aplayer-body:hover {
        left: 0px !important;
    }

    
</style>
<div class="">
    
    <div class="row">
        <meting-js class="col l8 offset-l2 m10 offset-m1 s12"
                   server="netease"
                   type="playlist"
                   id="7565528433"
                   fixed='true'
                   autoplay='false'
                   theme='#42b983'
                   loop='all'
                   order='random'
                   preload='auto'
                   volume='0.4'
                   list-folded='true'
        >
        </meting-js>
    </div>
</div>

<script src="/libs/aplayer/APlayer.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/meting@2/dist/Meting.min.js"></script>

    
    <div class="container row center-align" style="margin-bottom: 15px !important;">
        <div class="col s12 m8 l8 copy-right">
            Copyright&nbsp;&copy;
            
                <span id="year">2019-2024</span>
            
            <span id="year">2019</span>
            <a href="/about" target="_blank">YangChongZhi</a>
            |&nbsp;Powered by&nbsp;<a href="https://hexo.io/" target="_blank">Hexo</a>
            |&nbsp;Theme&nbsp;<a href="https://github.com/blinkfox/hexo-theme-matery" target="_blank">Matery</a>
            <br>
            
            &nbsp;<i class="fas fa-chart-area"></i>&nbsp;站点总字数:&nbsp;<span
                class="white-color">236.5k</span>&nbsp;字
            
            
            
            
            
            
            <span id="busuanzi_container_site_pv">
                |&nbsp;<i class="far fa-eye"></i>&nbsp;总访问量:&nbsp;<span id="busuanzi_value_site_pv"
                    class="white-color"></span>&nbsp;次
            </span>
            
            
            <span id="busuanzi_container_site_uv">
                |&nbsp;<i class="fas fa-users"></i>&nbsp;总访问人数:&nbsp;<span id="busuanzi_value_site_uv"
                    class="white-color"></span>&nbsp;人
            </span>
            
            <br>
            
            <span id="sitetime">载入运行时间...</span>
            <script>
                function siteTime() {
                    var seconds = 1000;
                    var minutes = seconds * 60;
                    var hours = minutes * 60;
                    var days = hours * 24;
                    var years = days * 365;
                    var today = new Date();
                    var startYear = "2019";
                    var startMonth = "6";
                    var startDate = "28";
                    var startHour = "0";
                    var startMinute = "0";
                    var startSecond = "0";
                    var todayYear = today.getFullYear();
                    var todayMonth = today.getMonth() + 1;
                    var todayDate = today.getDate();
                    var todayHour = today.getHours();
                    var todayMinute = today.getMinutes();
                    var todaySecond = today.getSeconds();
                    var t1 = Date.UTC(startYear, startMonth, startDate, startHour, startMinute, startSecond);
                    var t2 = Date.UTC(todayYear, todayMonth, todayDate, todayHour, todayMinute, todaySecond);
                    var diff = t2 - t1;
                    var diffYears = Math.floor(diff / years);
                    var diffDays = Math.floor((diff / days) - diffYears * 365);
                    var diffHours = Math.floor((diff - (diffYears * 365 + diffDays) * days) / hours);
                    var diffMinutes = Math.floor((diff - (diffYears * 365 + diffDays) * days - diffHours * hours) /
                        minutes);
                    var diffSeconds = Math.floor((diff - (diffYears * 365 + diffDays) * days - diffHours * hours -
                        diffMinutes * minutes) / seconds);
                    if (startYear == todayYear) {
                        document.getElementById("year").innerHTML = todayYear;
                        document.getElementById("sitetime").innerHTML = "本站已安全运行 " + diffDays + " 天 " + diffHours +
                            " 小时 " + diffMinutes + " 分钟 " + diffSeconds + " 秒";
                    } else {
                        document.getElementById("year").innerHTML = startYear + " - " + todayYear;
                        document.getElementById("sitetime").innerHTML = "本站已安全运行 " + diffYears + " 年 " + diffDays +
                            " 天 " + diffHours + " 小时 " + diffMinutes + " 分钟 " + diffSeconds + " 秒";
                    }
                }
                setInterval(siteTime, 1000);
            </script>
            
            <br>
            
            <span id="icp"><img src="/medias/icp.png" style="vertical-align: text-bottom;" />
                <a href="http://beian.miit.gov.cn/" target="_blank">渝ICP备2021001798号-1</a>
            </span>
            
        </div>
        <div class="col s12 m4 l4 social-link social-statis">
    <a href="https://github.com/yczlab" class="tooltipped" target="_blank" data-tooltip="访问我的GitHub" data-position="top" data-delay="50">
        <i class="fab fa-github"></i>
    </a>



    <a href="https://gitee.com/yczlab" class="tooltipped" target="_blank" data-tooltip="访问我的码云" data-position="top" data-delay="50">
        <i class="fab fa-github"></i>
    </a>



    <a href="mailto:2685733832@qq.com" class="tooltipped" target="_blank" data-tooltip="邮件联系我" data-position="top" data-delay="50">
        <i class="fas fa-envelope-open"></i>
    </a>







    <a href="tencent://AddContact/?fromId=50&fromSubId=1&subcmd=all&uin=2685733832" class="tooltipped" target="_blank" data-tooltip="QQ联系我: 2685733832" data-position="top" data-delay="50">
        <i class="fab fa-qq"></i>
    </a>



    <a href="tencent://AddContact/?fromId=50&fromSubId=1&subcmd=all&uin=2685733832" class="tooltipped" target="_blank" data-tooltip="微信联系我: yczlab" data-position="top" data-delay="50">
        <i class="fab fa-weixin"></i>
    </a>



    <a href="https://weibo.com/yczlab" class="tooltipped" target="_blank" data-tooltip="关注我的微博: https://weibo.com/yczlab" data-position="top" data-delay="50">
        <i class="fab fa-weibo"></i>
    </a>



    <a href="https://www.zhihu.com/people/yczlab" class="tooltipped" target="_blank" data-tooltip="关注我的知乎: https://www.zhihu.com/people/yczlab" data-position="top" data-delay="50">
        <i class="fab fa-zhihu1">知</i>
    </a>



</div>
    </div>
</footer>

<div class="progress-bar"></div>


    <!-- 搜索遮罩框 -->
<div id="searchModal" class="modal">
    <div class="modal-content">
        <div class="search-header">
            <span class="title"><i class="fas fa-search"></i>&nbsp;&nbsp;搜索</span>
            <input type="search" id="searchInput" name="s" placeholder="请输入搜索的关键字"
                   class="search-input">
        </div>
        <div id="searchResult"></div>
    </div>
</div>

<script type="text/javascript">
$(function () {
    var searchFunc = function (path, search_id, content_id) {
        'use strict';
        $.ajax({
            url: path,
            dataType: "xml",
            success: function (xmlResponse) {
                // get the contents from search data
                var datas = $("entry", xmlResponse).map(function () {
                    return {
                        title: $("title", this).text(),
                        content: $("content", this).text(),
                        url: $("url", this).text()
                    };
                }).get();
                var $input = document.getElementById(search_id);
                var $resultContent = document.getElementById(content_id);
                $input.addEventListener('input', function () {
                    var str = '<ul class=\"search-result-list\">';
                    var keywords = this.value.trim().toLowerCase().split(/[\s\-]+/);
                    $resultContent.innerHTML = "";
                    if (this.value.trim().length <= 0) {
                        return;
                    }
                    // perform local searching
                    datas.forEach(function (data) {
                        var isMatch = true;
                        var data_title = data.title.trim().toLowerCase();
                        var data_content = data.content.trim().replace(/<[^>]+>/g, "").toLowerCase();
                        var data_url = data.url;
                        data_url = data_url.indexOf('/') === 0 ? data.url : '/' + data_url;
                        var index_title = -1;
                        var index_content = -1;
                        var first_occur = -1;
                        // only match artiles with not empty titles and contents
                        if (data_title !== '' && data_content !== '') {
                            keywords.forEach(function (keyword, i) {
                                index_title = data_title.indexOf(keyword);
                                index_content = data_content.indexOf(keyword);
                                if (index_title < 0 && index_content < 0) {
                                    isMatch = false;
                                } else {
                                    if (index_content < 0) {
                                        index_content = 0;
                                    }
                                    if (i === 0) {
                                        first_occur = index_content;
                                    }
                                }
                            });
                        }
                        // show search results
                        if (isMatch) {
                            str += "<li><a href='" + data_url + "' class='search-result-title'>" + data_title + "</a>";
                            var content = data.content.trim().replace(/<[^>]+>/g, "");
                            if (first_occur >= 0) {
                                // cut out 100 characters
                                var start = first_occur - 20;
                                var end = first_occur + 80;
                                if (start < 0) {
                                    start = 0;
                                }
                                if (start === 0) {
                                    end = 100;
                                }
                                if (end > content.length) {
                                    end = content.length;
                                }
                                var match_content = content.substr(start, end);
                                // highlight all keywords
                                keywords.forEach(function (keyword) {
                                    var regS = new RegExp(keyword, "gi");
                                    match_content = match_content.replace(regS, "<em class=\"search-keyword\">" + keyword + "</em>");
                                });

                                str += "<p class=\"search-result\">" + match_content + "...</p>"
                            }
                            str += "</li>";
                        }
                    });
                    str += "</ul>";
                    $resultContent.innerHTML = str;
                });
            }
        });
    };

    searchFunc('/search.xml', 'searchInput', 'searchResult');
});
</script>

    <!-- 回到顶部按钮 -->
<div id="backTop" class="top-scroll">
    <a class="btn-floating btn-large waves-effect waves-light" href="#!">
        <i class="fas fa-arrow-up"></i>
    </a>
</div>


    <script src="/libs/materialize/materialize.min.js"></script>
    <script src="/libs/masonry/masonry.pkgd.min.js"></script>
    <script src="/libs/aos/aos.js"></script>
    <script src="/libs/scrollprogress/scrollProgress.min.js"></script>
    <script src="/libs/lightGallery/js/lightgallery-all.min.js"></script>
    <script src="/js/matery.js"></script>

    <!-- Baidu Analytics -->

    <!-- Baidu Push -->

<script>
    (function () {
        var bp = document.createElement('script');
        var curProtocol = window.location.protocol.split(':')[0];
        if (curProtocol === 'https') {
            bp.src = 'https://zz.bdstatic.com/linksubmit/push.js';
        } else {
            bp.src = 'http://push.zhanzhang.baidu.com/push.js';
        }
        var s = document.getElementsByTagName("script")[0];
        s.parentNode.insertBefore(bp, s);
    })();
</script>

    
    <script src="/libs/others/clicklove.js" async="async"></script>
    
    
    <script async src="/libs/others/busuanzi.pure.mini.js"></script>
    

    
        <script src="//code.tidio.co/ef3vil8uaqrvp7giqeg545xldzmymqss.js"></script>
    

    

	
    
    <script type="text/javascript" color="0,0,255"
        pointColor="0,0,255" opacity='0.5'
        zIndex="-1" count="99"
        src="/libs/background/canvas-nest.js"></script>
    

    
    
    <script type="text/javascript" size="150" alpha='0.6'
        zIndex="-1" src="/libs/background/ribbon-refresh.min.js" async="async"></script>
    

    
    <script type="text/javascript" src="/libs/background/ribbon-dynamic.js" async="async"></script>
    

    
    <script src="/libs/instantpage/instantpage.js" type="module"></script>
    
	
	<script type="text/javascript">
	var OriginTitile = document.title,st;
	document.addEventListener("visibilitychange", function () {
		document.hidden ? (document.title = "Σ(っ °Д °;)っ我隐藏了！", clearTimeout(st)) : (document.title="(๑•̀ㅂ•́) ✧被发现了！", st=setTimeout(function () {document.title = OriginTitile}, 3e3))
	});
	</script>
	
	<!-- 背景樱花飘落特效 -->
	
	
	<!-- 背景雪花飘落特效 -->
	
	
	<!-- 数字雨特效 -->
	
		<script type="text/javascript">
		//只在桌面版网页启用特效
		var windowWidth = $(window).width();
		if (windowWidth > 768) {
			document.write('<canvas id="digitalrain" width="1440" height="900" style="position:fixed; z-index:-2; right:0px; bottom:0px; min-width:100%; min-height:100%; height:auto; width:auto;"><\/canvas>');
			document.write('<script type="text/javascript" src="/js/digitalrain.js"><\/script>');
		}
		</script>
	

</body>

</html>
